U.S. application Ser. No. 10/126,026 of Alan C. Nelson, filed Apr. 19, 2002, entitled “VARIABLE-MOTION OPTICAL TOMOGRAPHY OF SMALL OBJECTS” is incorporated herein by this reference. In Nelson, projection images of shadowgrams are digitally captured by means of conventional image detectors such as CMOS or CCD detectors. In imaging moving objects, such image sensors require short exposures to “stop motion” in order to reduce motion blur. Short exposures limit the signal to noise ratio that can be attained when imaging moving objects.
Nelson's patent applications teach cone beam projection images or shadowgrams generated using sub-micron point sources of illumination and captured using CCD or CMOS image detectors. Cone beam illumination and projection geometry possesses the desirable characteristic that the transmitted projection image is magnified by virtue of the divergence, in two dimensions, or one dimension in the case of fan beam geometry, of the light ray paths in the beam. The aforesaid arrangement allows improvement of the resolution limitation that might otherwise be imposed by a detector pixel size, and the spatial resolution in the projections is ultimately limited by either the source aperture diameter or the wavelength of the illumination, whichever is greater.
Cone beam geometry for projection and tomographic imaging has been utilized in diagnostic and other x-ray imaging applications (Cheng, P C, Lin, T H, Wang, G, Shinozaki, D M, Kim, H G, and Newberry, S P, “Review on the Development of Cone-beam X-ray Microtomography”, Proceedings of the X-ray Optics and Microanalysis 1992, Institute of Physics Conference Series Volume 130, Kenway, P B, et al. (eds.), Manchester, UK, Aug. 31-Sep. 4, 1992, pp.559-66; Defrise, M, Clack, R, and Townsend, D W, “Image Reconstruction from Truncated, Two-dimensional, Parallel Projections”, Inverse Problems 11:287-313, 1995; Defrise, M, Noo, F, and Kudo, H, “A Solution to the Long-object Problem in Helical Cone-beam Tomography”, Physics in Medicine and Biology 45:623-43, 2000; Endo, M, Tsunoo, T, Nakamori, N, and Yoshida, K, “Effect of Scattered Radiation on Image Noise in Cone Beam CT”, Medical Physics 28(4):469-74, 2001; Taguchi, K and Aradate, H, “Algorithm for Image Reconstruction in Multi-slice Helical CT”, Medical Physics 25(4):550-61, 1998). There it arises naturally, since x-rays from thermally-assisted tungsten filament, electron-impact, laboratory or clinical diagnostic radiology sources invariably diverge from the point on the target anode that is bombarded by the accelerated electrons. Since the discovery of x-rays in 1895, the vast majority of x-ray sources have operated on the mechanisms of Bremsstrahlung and characteristic x-ray production. Except for synchrotrons, which are elaborate and expensive devices inaccessible to most research and healthcare professionals, parallel-beam x-ray sources are not available in the portions of the x-ray spectrum usually employed in clinical and scientific imaging applications. There are, however, lasers and other relatively inexpensive sources capable of producing intense, parallel-ray illumination in the visible and ultraviolet portions of the spectrum.
A number of researchers have employed parallel-beam geometry to perform synchrotron and laboratory x-ray microtomography (micro-CT). (See, for example, Bayat, S, Le Duc, G, Porra, L, Berruyer, G, Nemoz, C, Monfraix, S, Fiedler, S, Thomlinson, W, Suortti, P, Standertskjold-Nordenstam, C G, and Sovijarvi, A R A, “Quantitative Functional Lung Imaging with Synchrotron Radiation Using Inhaled Xenon as Contrast Agent”, Physics in Medicine and Biology 46:3287-99, 2001; Kinney, J H, Johnson, Q C, Saroyan, R A, Nichols, M C, Bonse, U, Nusshardt, R, and Pahl, R, “Energy-modulated X-ray Microtomography”, Review of Scientific Instruments 59(1):196-7, 1988. Kinney, J H and Nichols, M C, “X-ray Tomographic Microscopy (XTM) Using Synchrotron Radiation”, Annual Review of Material Science 22:121-52, 1992; Jorgensen, S M, Demirkaya, O, and Ritman, E L, “Three Dimensional Imaging of Vasculature and Parenchyma in Intact Rodent Organs with X-ray Micro-CT”, American Journal of Physiology 275(Heart Circ. Physiol. 44):H1103-14, 1998; Bentley, M D, Ortiz, M C, Ritman, E L, and Romero, J C, “The Use of Microcomputed Tomography to Study Microvasculature in Small Rodents”, American Journal of Physiology (Regulatory Integrative Comp Physiol) 282:R1267-R1279, 2002).
A syncrotron beam may be monochromatized using crystals or other optical elements from which it emerges with extremely low divergence. In the laboratory setting, with conventional microfocal x-ray sources, if the specimen or object is placed far from an intense x-ray source, it intercepts a relatively small cone of x-rays and the projection geometry may be approximated as parallel with only minimal detriment to the resulting image quality, though flux at the specimen is very low. Synchrotrons produce enormously intense radiation that facilitates relatively rapid scan times (e.g. scan times of seconds or minutes) for 3D microtomography. Unfortunately, synchrotron-based microtomography devices are very expensive. Electron-impact laboratory or clinical sources of the types described above are of much lower intensity relative to synchrotrons. In such systems, divergence of the beam and small cone angle subtended by a specimen placed remotely from the source in order to approximate the parallel geometry result in very low fluence at the specimen and commensurately long scan times of, for example, hours to days.
Although useful for various applications, cone beam projection geometry has some drawbacks. For example, the achievable spatial resolution is limited by the source size, thus mandating a sub-micron source for microscopic and cellular imaging. Further, the fluence or number of photons per unit area in the beam available from a sub-micron point source is very low, thereby placing stringent demands on the sensitivity and noise characteristics of the detector if adequate image quality and signal-to-noise ratio are to be obtained in the projection images. It is challenging to produce the sub-micron source size necessary to provide sub-micron resolution for cone beam imaging. Reproducibly fabricating such sub-micron light sources that produce relatively uniform or gaussian beam intensity profiles presents a significant challenge. For example, in some cases it is necessary to draw laser diode-pigtailed, single-mode optical fibers to a tapered tip. In other cases small apertures or microlenses must be placed between lasers or laser diodes or alternative light sources and the specimen. For optimal imaging and accurate image reconstruction, it is advantageous that the imaged object be positioned centrally in the cone beam, precisely aligned with the source position.
In the cone beam imaging geometry, projection magnification is strongly dependent upon the source-to-specimen distance, which is not the case in a parallel imaging geometry. In a dynamic flow tomographic imaging system, as described in the referenced Nelson patents, where the source-detector pairs may be disposed about a reconstruction cylinder in a variety of geometric arrangements, source-to-specimen distances must be precisely controlled and known to a high degree of accuracy for all source-detector pairs. Differing source-to-specimen distances between the source-detector pairs may result in degradation of the reconstructed image quality. Because projection magnification varies through the object space in cone beam imaging, the two-dimensional projection images or shadowgrams may be difficult to interpret. For example, it may be difficult to extract diagnostically-relevant features from the projection images directly. Cone beam projection geometry also requires 3D image reconstruction algorithms and computer programs that are complex and computationally intensive.